One of the most popular exercises is the invisible unicorn challenge. The children are told there are two invisible unicorns who live at Camp Quest but that they cannot be seen, heard, felt or smelt, and do not leave a trace. A book about them has been handed down through the ages but it is too precious for anyone to see.

All counsellors – as the adults are called – are said to be staunch believers in these unicorns.

Any child who can successfully prove that the invisible unicorns do not exist is rewarded with a prize: a £10 note with a picture of Charles Darwin on it signed by Richard Dawkins, or a "godless" $100 bill, printed before 1957 when "In God We Trust" was added to paper currency in the US.

Since this challenge began in 1996, the prize has been unclaimed.

The camp's director, Samantha Stein, said that the exercise had elicited all sorts of interesting responses from the children about the burden of proof. One child had insisted that it was up to the counsellors to prove the unicorns did exist. Another said it was just impossible to prove.

Stein said that the exercise was not about trying to bash the idea of God – just to make the children think critically and rationally.